Recovery of Valley Citrus Trees after the February Freeze
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BY DAVID LAUGHLIN AND VERONICA ANCONA, Plant Pathologists, Texas A&M University Kingsville Citrus Center
n February 14 of 2021, the Rio Grande Valley experienced the winter storm Uri that caused a severe freeze with temperatures dropping below freez ing for about 40 hours, reaching as low as 22 °F. In the days following the freeze, most citrus trees were in a poor state, losing practically all the leaves and flowers. Although Citrus are typically considered cold-tender plants, certain varieties such as kumquats and many mandarin varieties exhibit a greater tolerance to freeze damage while sweet orange and grapefruit tend to fall in the middle of the range and limes are the most sensitive to freezing temperatures. At the Texas A&M University Kingsville Citrus Center, we began evaluating the recovery of citrus trees 40 days following the freeze on three grapefruit, three sweet orange blocks and on a citrus variety block with 238 citrus varieties. Tree recovery was evaluated based on several parameters including the presence of new flush, flowering, bark splitting, damaged fruit and presence of pre-freezing leaves.
Similarly, the orange trees survived the freeze. No border effect was observed and the level of twig, branch and bark splitting damage in the orange blocks was remarkably low. All of the trees were flushing vigorously and flowering. Unfortunately, all of the fruit that was not harvested had dropped to the soil due to the freeze (Figure 3).
Figure 2. Freeze damage on citrus branches. Severe (left) and mild (right) bark splitting on grapefruit. Most grapefruit trees showed some degree of bark splitting compared to sweet oranges and mandarins.
All of the grapefruit trees evaluated were alive. There was a noticeable border effect in which grapefruit trees on the edge of the blocks were more severely freeze damaged (Figure 1). Even trees with extensive damage were exhibiting various levels of flush and most were flowering. Except for trees near the edges of the block, many of the grapefruit trees still had undamaged fruit hanging on the tree. Even though the trees were alive and flushing there was still extensive canopy dieback and bark splitting (Figure 2).
Figure 3. Extensive fruit drop occurred in the weeks following the freeze
Figure 1. Aerial photographs of mature Rio Red grapefruit block in 2019 (top) and April 2021 (bottom). The border effect can clearly be seen as the trees in the center of the block are greener. Image courtesy of Jorge Solorzano and Juan Enciso, AgriLife Research.
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In the variety block at the TAMUK Citrus Center we observed different levels of tree recovery (Figure 4). Most of the grapefruit varieties including old-line white and pink varieties showed moderate levels of freeze damage. Many of the trees still had undamaged fruit still hanging on the trees and were flushing and flowering. The sweet orange varieties including navels/early, mid-season, late, and blood oranges fared reasonably well and some were blooming vigorously (Figure 5). However, none of the sweet oranges had undamaged fruit on the tree. Mandarin oranges displayed the greatest range in freeze response compared to the other citrus types, likely because is the most genetically diverse group. Like the orange group, the mandarins had a large percentage of trees recovering vigorously, although some varieties were severely damaged. The pomelo and lemon vari-
